I am no expert, but I feel I am pretty good at the early game.

This is what I focus on and in the order I do them.

• When you start you want 2 diggers/builders and 1 research dupe.
• Pause the game and have a look around. I want to see where I want to put the basics of the base. Plan on levels with 4 block high of space.
• You want a vertical shaft going through the entire base. With a ladder a 1 empty space on either side. On of those spaces will hold a fire pole.
• Dig a latrine room. Plan a space for a door, 8 spaces for wash basins, and then build 2 outhouses.
• Dig a bedroom. Plan a space for 8 beds and a door on either side. Build 3 beds.
• Start digging towards your water source and build 2 wash basins in the latrine. This is probably the end of the first day. Your dupe can go to the bathroom and sleep in a bed.
• Build the rooms blocking in the latrine and bedroom. Tomorrow, your dupes morale will be higher.
• Build a O2 diffusser one 1 level below the gate. Build a treadmill and 2 batteries next to it. Increase priority of built diffuser to get it running.
• Build research station right next to gate. Gate emits light and that will make research happen 15% faster.
• Build a storage bin near the research station and fill it with dirt. The researcher will do nothing else until there is nothing left to research.
• Start researching for farm tiles. You will want to get into mealwood farming. Food for dirt, good trade.
• Dig out the area, with your long term base in mind. You will find Muckroot to feed your dupes and you will find seeds.

I recommend the wiki and Francis John on YouTube. A new up and coming source is Blueprint not included for good ideas.

I don't go into slime biomes until I am ready to get ready for Atmosuits. Once I am using Atmosuits, don't go anywhere outside my main base without them.